Unified Cascaded Encoder ASR model for Dynamic Model Sizes

ABSTRACT

An automated speech recognition (ASR) model includes a first encoder, a first encoder, a second encoder, and a second decoder. The first encoder receives, as input, a sequence of acoustic frames, and generates, at each of a plurality of output steps, a first higher order feature representation for a corresponding acoustic frame in the sequence of acoustic frames. The first decoder receives, as input, the first higher order feature representation generated by the first encoder, and generates a first probability distribution over possible speech recognition hypotheses. The second encoder receives, as input, the first higher order feature representation generated by the first encoder, and generates a second higher order feature representation for a corresponding first higher order feature frame. The second decoder receives, as input, the second higher order feature representation generated by the second encoder, and generates a second probability distribution over possible speech recognition hypotheses.

CROSS REFERENCE TO RELATED APPLICATIONS

This U.S. patent application claims priority under 35 U.S.C. §119(e) to U.S. Provisional Application 63/269,703, filed on Mar. 21, 2022. The disclosure of this prior application is considered part of the disclosure of this application and is hereby incorporated by reference in its entirety.

TECHNICAL FIELD

This disclosure relates to an on-device end-to-end model for simplified streaming and non-streaming automated speech recognition (ASR).

BACKGROUND

Automated speech recognition (ASR) systems have evolved from multiple models where each model had a dedicated purpose to integrated models where a single neural network is used to directly map an audio waveform (i.e., input sequence) to an output sentence (i.e., output sequence). This integration has resulted in a sequence-to-sequence approach, which generates a sequence of words (or graphemes) when given a sequence of audio features. With an integrated structure, all components of a model may be trained jointly as a single end-to-end (E2E) neural network. Here, an E2E model refers to a model whose architecture is constructed entirely of a neural network. A fully neural network functions without external and/or manually designed components (e.g., finite state transducers, a lexicon, or text normalization modules). Additionally, when training E2E models, these models generally do not require bootstrapping from decision trees or time alignments from a separate system. These E2E automatic speech recognition (ASR) systems have made tremendous progress, surpassing conventional ASR systems in several common benchmarks including word error rates (WER). The architecture of E2E ASR models are largely application dependent. For instance, a number of applications that involve user interaction, such as voice-search or on-device dictation, require the model to perform recognition in a streaming fashion. Other applications, like offline video captioning, do not require the model to be streaming and can make use of future context to improve performance. Moreover, the model size for an on-device ASR system is largely dependent on the hardware constraints of the device. For example, vehicle devices equipped with processing hardware with greater computational output may support a larger ASR model size, whereas a mobile device with limited processing hardware may benefit from a smaller ASR model size. Existing E2E approaches typically include separate ASR models for streaming and non-streaming applications. Managing multiple models for different applications provides unique challenges and can lead to slower execution times of the ASR systems.

SUMMARY

One aspect of the present disclosure provides an automatic speech recognition (ASR) model that includes a first encoder, a first encoder, a second encoder, and a second decoder. The first encoder receives, as input, a sequence of acoustic frames, and generates, at each of a plurality of output steps, a first higher order feature representation for a corresponding acoustic frame in the sequence of acoustic frames. The first decoder receives, as input, the first higher order feature representation generated by the first encoder at each of the plurality of output steps, and generates, at each of the plurality of output steps, a first probability distribution over possible speech recognition hypotheses. The second encoder receives, as input, the first higher order feature representation generated by the first encoder at each of the plurality of output steps, and generates, at each of the plurality of output steps, a second higher order feature representation for a corresponding first higher order feature frame. The second decoder receives, as input, the second higher order feature representation generated by the second encoder at each of the plurality of output steps, and generates, at each of the plurality of output steps, a second probability distribution over possible speech recognition hypotheses.

Implementations of the disclosure may include one or more of the following optional features. In some implementations, the first decoder generates partial speech recognition results based on the first probability distribution over possible speech recognition hypotheses. In some examples, the first encoder includes a causal encoder including one of a plurality of unidirectional long short-term memory (LSTM) layers, a plurality of conformer layers, or a plurality of transformer layers. Additionally or alternatively, the second encoder includes a non-causal encoder including one of a plurality of unidirectional long short-term memory (LSTM) layers, a plurality of conformer layers, or a plurality of transformer layers.

In some examples, the first decoder includes a prediction network and a joint network. The prediction receives, as input, a sequence of non-blank symbols output by a final softmax layer, and generates, at each of the plurality of output steps, a dense representation. The joint network receives, as input, the dense representation generated by the prediction network at each of the plurality of output steps and the first higher order feature representation generated by the first encoder at each of the plurality of output steps, and generates, at each of the plurality of output steps, the first probability distribution over possible speech recognition hypotheses. In these examples, the prediction network includes a long short-term memory (LSTM)-based prediction network, or a V2 embedding look-up table.

In some implementations, the second decoder includes a prediction network and a joint network. The prediction network receives, as input, a sequence of non-blank symbols output by a final softmax layer, and generates, at each of the plurality of output steps, a dense representation. The joint network receives, as input, the dense representation generated by the prediction network at each of the plurality of output steps and the second higher order feature representation generated by the second encoder at each of the plurality of output steps, and generates, at each of the plurality of output steps, the second probability distribution over possible speech recognition hypotheses. In these implementations, the prediction network includes a long short-term memory (LSTM)-based prediction network, or a V2 embedding look-up table.

In some examples, the first encoder includes a greater number of parameters than the second encoder. In some implementations, the ASR model further includes a third encoder and a third decoder. The third encoder receives, as input, the second higher order feature representation generated by the second encoder at each of the plurality of output steps, and generates, at each of the plurality of output steps, a third higher order feature representation for a corresponding second higher order feature representation. The third decoder receives, as input, the third higher order feature representation generated by the third encoder at each of the plurality of output steps, and generates, at each of the plurality of output steps, a third probability distribution over possible speech recognition hypotheses.

Another aspect of the present disclosure provides a computer-implemented method that when executed on data processing hardware causes the data processing hardware to perform operations that include receiving, as input to an automatic speech recognition (ASR) model, a sequence of acoustic frames, and performing, using the ASR model, streaming speech recognition and non-streaming speech recognition on the sequence of acoustic frames. The operations also include generating, by a first encoder, at each of a plurality of output steps, a first higher order feature representation for a corresponding acoustic frame in the sequence of acoustic frames, and receiving, as input to both a first decoder and a second encoder, the first higher order feature representation generated by the first encoder at each of the plurality of output steps. The operations also include generating, by the second encoder, at each of the plurality of output steps, a second higher order feature representation for a corresponding first higher order feature representation, and generating, by the first decoder, at each of the plurality of output steps, a first probability distribution over possible speech recognition hypotheses. The operations further include receiving, as input to a second decoder, the second higher order feature representation generated by the second encoder at each of the plurality of output steps, and generating, by the second decoder, at each of the plurality of output steps, a second probability distribution over possible speech recognition hypotheses.

This aspect may include one or more of the following optional features. In some implementations, the operations further include generating partial speech recognition results based on the first probability distribution over possible speech recognition hypotheses. In some examples, the first encoder includes a causal encoder including one of a plurality of unidirectional long short-term memory (LSTM) layers, a plurality of conformer layers, or a plurality of transformer layers. Additionally or alternatively, the second encoder includes a non-causal encoder including one of a plurality of unidirectional long short-term memory (LSTM) layers, a plurality of conformer layers, or a plurality of transformer layers.

In some implementations, when the ASR model is operating in a streaming mode, the operations further include receiving, as input to a prediction network of the first decoder, a sequence of non-blank symbols output by a final softmax layer, generating, by the prediction network of the first decoder, at each of the plurality of output steps, a dense representation. Here, the operations further include receiving, as input to a joint network of the first decoder, the dense representation generated by the prediction network at each of the plurality of steps and the first higher order feature representation generated by the first encoder at each of the plurality of output steps, and generating, by the joint network of the first decoder, at each of the plurality of output steps, the first probability distribution over possible speech recognition hypotheses. In these implementations, the prediction network of the first decoder includes a long short-term memory (LSTM)-based prediction network, or a V2 embedding look-up table.

In some examples, when the ASR model is operating in a non-streaming mode, the operations also include receiving, as input to a prediction network of the second decoder, a sequence of non-blank symbols output by a final softmax layer, and generating, by the prediction network of the second decoder, at each of the plurality of output steps, a dense representation. Here, the operations further include receiving, as input to a joint network of the second decoder, the dense representation generated by the prediction network at each of the plurality of output steps and the second higher order feature representation generated by the second encoder at each of the plurality of output steps, and generating, by the joint network of the second decoder, at each of the plurality of output steps, the second probability distribution over possible speech recognition hypotheses. In these examples, the prediction network of the second decoder includes a long short-term memory (LSTM)-based prediction network, or a V2 embedding look-up table.

In some implementations, the first encoder includes a greater number of parameters than the second encoder. In some examples, the operations also include, while performing the streaming speech recognition and the non-streaming speech recognition on the sequence of acoustic frames, receiving, as input to a third encoder, the second higher order feature representation generated by the second encoder at each of the plurality of output steps. In these implementations, the operations further include generating, by the third encoder, at each of the plurality of output steps, a third higher order feature representation for a corresponding second higher order feature representation, receiving, as input to a third decoder, the third higher order feature representation generated by the third encoder at each of the plurality of output steps, and generating, by the third decoder, at each of the plurality of output steps, a third probability distribution over possible speech recognition hypotheses.

The details of one or more implementations of the disclosure are set forth in the accompanying drawings and the description below. Other aspects, features, and advantages will be apparent from the description and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIGS. 1A and 1B are schematic views of a system using a cascaded encoders model architecture for automatic speech recognition.

FIGS. 2A-2D are schematic views of the cascaded encoders model architecture.

FIGS. 3A and 3B are schematic views of an example training process for promoting a cascaded encoders model to learn consistent predictions on both streaming automatic speech recognition and non-streaming automatic speech recognition.

FIG. 4 is a flowchart of an example arrangement of operations for a method of implementing the cascaded encoders model for both streaming automatic speech recognition and non-streaming automatic speech recognition.

FIG. 5 is a schematic view of an example computing device that may be used to implement the systems and methods described herein.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

End-to-end (E2E) automatic speech recognition (ASR) models are traditionally structured to operate in either a streaming mode or a non-streaming mode. Conventionally, an E2E ASR model includes an encoder and a decoder as the main components. Applications that involve end-user interaction, like voice-search or on-device dictation, may require the model to perform recognition in a streaming fashion, where the words are expected to be output as they are spoken with as little latency as possible. This prevents the use of models that use future context to improve accuracy, such as bi-directional LSTMs. By contrast, applications such as offline video captioning do not require streaming recognition and may make full use of any available future context to improve performance. Moreover, the model size for an on-device ASR system is largely dependent on the hardware constraints of the device. For example, vehicle devices equipped with processing hardware with greater computational output may support a larger ASR model size, whereas a mobile device with limited processing hardware may benefit from a smaller ASR model size. Even on the same device, a large ASR model size may be used for short-form applications such as voice search, whereas a medium ASR model size or small ASR model size may be better suited for long-running applications such as dictation or video captioning to maintain low power consumption.

Implementations herein are directed toward a single E2E ASR model that uses cascaded encoders and two separate decoders to enable operation in both streaming and non-streaming modes with varying model parameters. The cascaded encoders include a streaming encoder and a non-streaming encoder, and the two separate decoders include a streaming decoder and a non-streaming decoder that are configured to learn to decode the output from the respective streaming encoder or the output from the respective non-streaming encoder. In addition to ASR models, the architecture can apply to other models such as machine translation that implement both streaming and non-streaming modes.

Referring to FIGS. 1A and 1B in some implementations, a system 100, 100 a-b includes a user 104 interacting with speech-enabled user device 10 (also referred to as a device 10 or a user device 10) within a speech environment. In the system 100, the user’s 104 manner of interacting with the user device 10 may be through voice input. The user device 10 is configured to capture sounds (e.g., streaming audio data) from one or more users 104 within the speech environment. Here, the streaming audio data may refer to a spoken utterance 106 by the user 104 that functions as an audible query, a command for the device 10, or an audible communication captured by the device 10. Speech-enabled systems of the device 10 may field the query or the command by answering the query and/or causing the command to be performed/fulfilled by one or more downstream applications.

The user device 10 may correspond to any computing device associated with a user 104 and capable of receiving audio data. Some examples of user devices 10 include, but are not limited to, mobile devices (e.g., mobile phones, tablets, laptops, etc.), computers, wearable devices (e.g., smart watches), smart appliances, internet of things (IoT) devices, vehicle infotainment systems, smart displays, smart speakers, etc. The user device 10 includes data processing hardware 12 and memory hardware 14 in communication with the data processing hardware 12 and stores instructions, that when executed by the data processing hardware 12, cause the data processing hardware 12 to perform one or more operations. The user device 10 further includes an audio system 16 with an audio capture device (e.g., microphone) 16, 16 a for capturing and converting spoken utterances 106 within the speech environment into electrical signals and a speech output device (e.g., a speaker) 16, 16 b for communicating an audible audio signal (e.g., as output audio data from the device 10). While the user device 10 implements a single audio capture device 16 a in the example shown, the user device 10 may implement an array of audio capture devices 16 a without departing from the scope of the present disclosure, whereby one or more capture devices 16a in the array may not physically reside on the user device 10, but be in communication with the audio system 16.

In the system 100, an automated speech recognition (ASR) system 109 implementing an ASR model 200 (also referred to as the model 200) resides on the user device 10 of the user 104 and/or on a remote computing device 60 (e.g., one or more remote servers of a distributed system executing in a cloud-computing environment) in communication with the user device 10 via a network 40. The remote computing device 60 may include remote data processing hardware 62 (e.g., remote servers or CPUs) and/or remote memory hardware 64 (e.g., remote databases or other storage hardware). The user device 10 and/or the remote computing device 60 also includes an audio subsystem 108 configured to receive the utterance 106 spoken by the user 104 and captured by the audio capture device 16 a, and to convert the utterance 106 into a corresponding digital format associated with input acoustic frames 110 capable of being processed by the ASR system 109. In the example shown in FIG. 1A, the user 104 speaks a respective utterance 106 and the audio subsystem 108 converts the utterance 106 into corresponding audio data (e.g., acoustic frames) 110 for input to the ASR system 109. Thereafter, the model 200 receives, as input, the audio data 110 corresponding to the utterance 106, and generates/predicts, as output, a corresponding transcription 120 (also referred to as a recognition result/hypothesis 120) of the utterance 106. As described in greater detail below (e.g., FIGS. 3A and 2B), the model 200 may be trained in two-stage training to simplify the process of training the model 200 to operate in a streaming and a non-streaming mode. The model 200 also includes a first decoder 204 (also referred to as a streaming decoder 204) and a second decoder 206 (also referred to as a non-streaming decoder 206) which enable the model 200 to operate in a streaming mode and a non-streaming mode (e.g., in contrast with two separate models where each model is dedicated to either a streaming mode or non-streaming mode).

For instance, as shown in FIG. 1A, a digital assistant application 50 executing on the user device 10 may require the speech recognition to be streaming such that words, word pieces, and/or individual characters appear on the screen as soon as they are spoken. Additionally, it is also likely that the user 104 of the user device 10 has a low tolerance for latency when issuing queries for the digital assistant application 50 to perform. In these scenarios where the application demands minimal latency and benefits from a small model size, the model 200 operates in a streaming mode where the model 200 may provide streaming transcription capabilities in real-time as the user 104 is speaking the utterance 106. On the other hand, when the user 104 has a higher tolerance for speech recognition latency and/or the utterance 106 to be recognized is associated with long-form speech (i.e., referring to speech consisting of full paragraphs or multiple sentences) and the capacity for a larger model, the same model 200 may operate in a non-streaming mode and may leverage a prediction network to provide an accurate transcription 120, but incur increased latency. Moreover, in scenarios where a user 104 has an even higher tolerance for speech recognition latency and/or the accuracy is of the utmost importance, the model 200 may operate in a non-streaming mode that uses additional context to improve accuracy at the cost of increased speech recognition latency. Accordingly, the ASR system 109 may implement the ASR model 200 for a multitude of different speech recognition tasks to provide both streaming and non-streaming transcription capabilities without having to leverage separately trained ASR models on a task-by-task basis. Furthermore, using the single ASR model 200 for varying speech recognition tasks reduces the computational requirements to operate separate ASR models.

In some implementations, the model 200 performs streaming speech recognition on the audio data 110 first and then performs non-streaming speech recognition on the output of the streaming encoder. For instance, in the example shown, the model 200 performs streaming speech recognition on the audio data 110 using a first encoder (i.e., a low latency encoder (FIG. 2B)) to produce partial speech recognition results 120, 120 a, and non-streaming speech recognition on the encoded audio data 110 using a second encoder (i.e., a high latency encoder (FIG. 2C)) to produce a final speech recognition result 120, 120 b. Notably, the model 200 outputs the partial speech recognition results 120 a as the audio data 110 is received and subsequently outputs the final speech recognition result 120 b once all the audio data 110 is received after the user 104 finishes speaking the utterance 106. Thus, output of the final speech recognition result 120 b for the input utterance 106 may be delayed from the partial speech recognition results 120 a.

The user device 10 and/or the remote computing device 60 also executes a user interface generator 107 configured to present a representation of the transcription 120 of the utterance 106 to the user 104 of the user device 10. As described in greater detail below, the user interface generator 107 may display the partial speech recognition results 120 a in a streaming fashion during time 1 and subsequently display the final speech recognition result 120 b during time 2. In some configurations, the transcription 120 output from the ASR system 109 is processed, e.g., by a natural language understanding (NLU) module executing on the user device 10 or the remote computing device 60, to execute a user command/query specified by the utterance 106. Additionally or alternatively, a text-to-speech system (not shown) (e.g., executing on any combination of the user device 10 or the remote computing device 60) may convert the transcription 120 into synthesized speech for audible output by the user device 10 and/or another device.

In the example system 100 a of FIG. 1A, the user 104 in the speech environment interacts with a program or application 50 (e.g., the digital assistant application 50 a) of the user device 10 that uses the ASR system 109. For instance, FIG. 1A depicts the user 104 communicating with the digital assistant application 50 a and the digital assistant application 50 a displaying a digital assistant interface 18 on a screen of the user device 10 to depict a conversation between the user 104 and a digital assistant of the digital assistant application 50 a. In this example, the user 104 asks the digital assistant application 50 a, “What song is playing right now?” This question from the user 104 is a spoken utterance 106 captured by the audio capture device 16 a and processed by audio systems 16 of the user device 10. In this example, the audio system 16 receives the spoken utterance 106 and converts it into acoustic frames 110 for input to the ASR system 109.

Continuing with the example, the model 200, while receiving the acoustic frames 110 corresponding to the utterance 106 as the user 104 speaks, encodes the acoustic frames 110 using a first encoder 210 (i.e., FIGS. 2A and 2B) and then decodes an encoded representation of the acoustic frames 110 using a first decoder 204 (FIGS. 2A and 2B) into the partial speech recognition results 120 a. During time 1, the user interface generator 107 presents, via the digital assistant interface 18, a representation of the partial speech recognition results 120 a of the utterance 106 to the user 104 of the user device 10 in a streaming fashion such that words, word pieces, and/or individual characters appear on the screen as soon as they are spoken.

After all (or some amount) of the acoustic frames 110 corresponding to the utterance 106 are received, and the first encoder 210 has encoded these acoustic frames 110, the second encoder 220 (i.e., FIGS. 2A and 2C) encodes the encoding output from the first encoder 210 to generate an encoding for the set of acoustic frames 110 corresponding to the utterance 106 already encoded by the first encoder 210. A second decoder 206 (i.e., FIGS. 2A and 2C) then decodes the acoustic frames 110 that have been encoded by the second encoder 220 into a final speech recognition result 120 b. For example, when the first encoder 210 encodes all of the acoustic frames 110 corresponding to the utterance 106 (e.g., as the acoustic frames 110 are received), the second encoder 220 encodes all of the acoustic frames 110 that have been encoded by the first encoder 210. In this respect, by encoding over multiple encoded acoustic frames 110, the second encoder 220 is able to provide greater contextual awareness (e.g., by receiving representations of all of the acoustic frames 110 for the utterance 106) in a non-streaming fashion which may potentially reconcile or correct aspect(s) of the utterance 106 missed or misinterpreted by the streaming nature of the first encoder 210. In some examples, an indication, such as an endpoint, that identifies that the user 104 has finished speaking the utterance 106 triggers the second encoder 220 of the model 200 to encode all the acoustic frames 110.

During time 2, the user interface generator 107 presents, via the digital assistant interface 18, a representation of the final speech recognition result 120 b of the utterance 106 to the user 104 of the user device 10. In some implementations, the user interface generator 107 replaces (or modifies) the representation of the partial speech recognition results 120 a with the representation of the final speech recognition result 120 b. For instance, the final speech recognition result 120 b is presumed to be more accurate than the partial speech recognition results 120 a and the final speech recognition result 120 b is ultimately displayed as the transcription 120 in order to fix any terms that may have been misrecognized in the partial speech recognition results 120 a. In this example, the streaming partial speech recognition results 120 a output by the model 200 (i.e., the first encoder 210 and the first decoder 204) and displayed on the screen of the user device 10 at time 1 are associated with low latency and provide responsiveness to the user 104 that his/her query is being processed. Accordingly, the partial speech recognition results 120 a may incorrectly predict that the utterance 106 of the user 104 is “What song is play right now?” The final speech recognition result 120 b output by the model 200 (i.e., the cascaded encoders 202 and the second decoder 206) and displayed on the screen at time 2 at increased latency improves the speech recognition quality in terms of accuracy by identifying that the user 104 said “playing.” However, since the user interface generator 107 displays the partial speech recognition results as the user speaks the utterance 106, the higher latency associated with producing, and ultimately displaying the final recognition result 120 b is less noticeable to the user 104.

In the example shown in FIG. 1A, the digital assistant application 50 a may respond to the question posed by the user 104 using natural language processing. Natural language processing generally refers to a process of interpreting written language (e.g., the partial speech recognition results 120 a and/or the final speech recognition result 120 b) and determining whether the written language prompts any action. In this example, the digital assistant application 50 a uses natural language processing to recognize that the question from the user 104 regards the user’s environment and more particularly a song playing in the user’s vicinity. By recognizing these details with natural language processing, the automated assistant returns a response 19 to the user’s query where the response 19 states, “Cloudplay is playing right now.” In some configurations, natural language processing occurs on the remote computing device 60 in communication with the data processing hardware 12 of the user device 10.

FIG. 1B is another example of speech recognition with the ASR system 109 of the speech environment 100 b. As shown in the example, the user 104 interacts with a voicemail application 50, 50 b displaying a voicemail application interface 18, 18 b on the screen of the user device 10 to transcribe a voicemail that was left for the user 104 by Jane Doe. In this example, latency is not important. Without a concern for latency, the model 200 of the ASR system 109 is able to take advantage of the full context of the audio by waiting until all of the acoustic frames 110 corresponding to the voicemail are generated. This voicemail scenario also illustrates how the model 200 is capable of handling a long-form of speech because a voicemail is often multiple sentences or even several paragraphs. The ability to handle long-form speech is particularly advantageous over other ASR models, such as multi-pass models with LAS decoders, because these models often incur reduced performance (e.g., a higher word deletion rate on long-form speech) when recognizing long-form speech. For instance, by using a dedicated second decoder 206 trained on the features of the second encoder 220 in combination with cascaded encoders 202 (e.g., the first encoder 210 and the second encoder 220), the model 200 is able to consume the full context of the input audio when performing speech recognition on long-form speech without incurring a reduction performance. As discussed in further detail below, the cascading encoders 202 of the model 200 may optionally include a third encoder 230 in cascade with the first encoder 210 and the second encoder 220, and a dedicated third decoder 208 that decodes the acoustic frames 110 that have been encoded by the third encoder 230 into the final speech recognition result 120 b.

With continued reference to FIG. 1B, as discussed with respect to FIG. 1A, the model 200 encodes the acoustic frames 110 using the first encoder 210 while receiving the acoustic frames 110. After the model 200 receives all of the acoustic frames 110 and encodes them with the first encoder 210, the model 200 provides the first encoder output as input to the second encoder 220. The second encoder 220 encodes the first encoder output before the second decoder 206 generates the final speech recognition result 120 b. Thereafter, the user interface generator 107 presents, via the digital assistant interface 18 b, a representation of the final speech recognition result 120 b without first displaying the partial speech recognition results 120 a. For example, the final speech recognition result 120 b is a transcript of the long-form voicemail from Jane Doe that states “Give me a call back when you get this. Just trying to figure out plans for the New Year.”

FIGS. 2A-2D include example models 200 a-d operating in various combinations of streaming and non-streaming modes. Specifically, each of the models 200 a-d include a cascading encoder 202, a first decoder 204, and a second decoder 206. The cascaded encoder 202 refers to a model structure where the encoding pathway includes the encoders 210, 220 that cascade such that the output of one encoder 210 feeds the input of a second encoder 220 prior to decoding. Here, the encoders 210, 220 can be cascaded irrespective of the underlying architecture for each encoder. As shown in FIG. 2D, and discussed in more detail below, the model 200 can be generalized to a three-pass model 200 (i.e., a large model) where the cascaded encoder 202 includes a third encoder 230 in cascade with the encoders 210, 220 and connected to a dedicated third decoder 208. Here, the output of the second encoder 220 feeds the input of the third encoder 230 prior to decoding by the third decoder 208.

In some examples, the encoders 210, 220, 230 include a stack of 512-dimension conformer layers. In other examples, the first encoder 210 (i.e., a first pass) includes a stack (e.g., six layers) of 256-dimension causal conformer layers, the second encoder 220 (i.e., a second pass) includes a stack (e.g., six layers) of 512-dimension non-causal conformer layers, and the third encoder 230 (i.e., the third pass) includes a stack (e.g., six layers) of 640-dimension non-casual conformer layers. Causal convolution and left-context attention layers may be used for each conformer layer to strictly restrict the model to use no future inputs. A multi-headed (e.g., 8 heads) attention mechanism may be used in a self-attention layer.

The encoder 210, when the model 200 operates as a first pass model, includes six conformer layers with around 20 million parameters. The cascaded encoders 210, 220, when the model 200 operates as the second pass model, may include 12 conformer layers with around 50 million parameters. Here, the first encoder 210 may include six conformer layers with around 20 million parameters, where the first three layers have no self-attention (e.g., a 92 frame left context and no right context) to prevent the model 200 from using future inputs, while the second encoder 220 may include six conformer layers that take in additional right context (e.g., 5.04 seconds) with around 30 million parameters. The cascaded encoders 210, 220, 230, when the model 200 operates as a third pass model, includes 18 layers with around 110 million parameters, where the third encoder 230 includes six non-causal conformer layers with an additional right context (e.g., 30 frame right context of 900 milliseconds (ms) with around 60 million parameters. Optionally, other types of layers incorporating self-attention mechanisms, such as transformer layers, may be used in lieu of conformer layers. The first encoder 210 may be referred to as a causal encoder and the second encoder 220 and third encoder 230 may each be referred to as a non-causal encoder.

In other implementations, one encoder is constructed with an LSTM structure while the other encoder is constructed using bi-directional LSTM layers or conformer layers (e.g., a conformer-transducer). In other words, the encoders 210, 220, 230 may have different architectures or similar architectures. For instance, the cascading encoder 202 may be roughly analogous to an acoustic model (AM) in a traditional ASR system, and may include a recurrent network of stacked Long Short-Term Memory (LSTM) layers. Here, the first encoder 210 is a streaming encoder that includes unidirectional Long Short Term Memory (LSTM) layers while the second encoder 220 is a non-streaming encoder that includes bidirectional LSTM layers or conformer layers. In a cascading encoder 202, where the encoders 210, 220, 230 include LSTM layers, the second encoder 220 that receives the output of the first encoder 210 may take advantage of the LSTM layers of the first encoder 210 such that the second encoder 220 includes fewer LSTM layers than the first encoder 210 (and fewer LSTM layers than a fully non-streaming model). By having fewer LSTM layers, the cascaded encoder 202 may reduce the number of more computationally expensive bidirectional layers making the model 200 more streamlined than simply combining a traditional streaming model with a traditional non-streaming model.

Referring to FIG. 2A, the first encoder 210 reads a sequence of d-dimensional feature vectors (e.g., acoustic frames 110 shown in FIGS. 1A and 1B) x = (x₁, x₂, · · ·, x_(T)), where x_(t) ∈ ℝ^(d), and produces, at each output step, a first higher-order feature representation. This first higher-order feature representation is denoted as e^(S). Similarly, the second encoder 220 is connected in cascade to the first encoder 210, and is trained to receive the first higher order feature e^(S) as input, and output a second higher order feature representation. This second higher order feature representation is denoted as e^(a). Optionally, the third encoder 230 is connected in cascade to the second encoder 220, and is trained to receive the second higher order feature representation e^(a) as input, and output a third higher order feature representation. This third higher order feature representation is denoted as e^(r). Each of the first encoder 210, the second encoder 220, and the third encoder 230 are directly connected to their own dedicated, smaller decoders 204, 206, 208. Specifically, the first encoder 210 is connected to the first decoder 204, the second encoder 220 is connected to the second decoder 206, and the third encoder 230 is connected to the third decoder 208. Accordingly, the first decoder 204 receives the first higher order feature representation e^(S) input, the second decoder 206 receives the second higher order feature representation e^(a) as input, and the third decoder 208 receives the third higher order feature representation e^(r) as input.

The first decoder 204, the second decoder 206, and the third decoder 208 may each include a recurrent neural network-transducer (RNN-T) architecture having a respective 4.4 million parameters based on a 384-dimensional fully-connected joint layer 240, 240 a-c and a respective 320-dimensional embedding prediction network 250, 250 a-c. When the model 200 operates in the streaming mode (i.e., FIG. 2B), the joint layer 240 a of the first decoder 204 receives an embedding output from the prediction network 250 a (for the previous prediction y_(r-1)) and combines it with the first higher order feature representation e^(S) output from the first encoder 210 in order to produce the first decoder 204 output. When the model 200 operates in the two-pass non-streaming mode (FIG. 2C), the second decoder 206 uses the joint layer 240 b to combine the second higher order feature representations e^(a) output by the cascaded encoder 202, as well as an embedding output from the prediction network 250 b (for the previous prediction y_(r-1)), in order to produce the second decoder 206 output. Similarly, when the model 200 operates in the third-pass non-streaming mode (FIG. 2D), the third decoder 208 uses the joint layer 240 c to combine the third higher order feature representation e^(r) output by the cascaded encoder 202, as well as an embedding output from the prediction network 250 c (for the previous prediction y_(r-) ₁), in order to produce the third decoder 208 output..

The respective outputs of the decoders 204, 206, 208 can be a probability distribution, P (y_(i)|y_(i-1), ..., y₀, x), over the current sub-word unit, y_(i), given the sequence of the N previous non-blank symbols previous units, {y_(i-1), ..., y_(i-N)}, and input, x. Although not illustrated, the model 200 may include a Softmax layer that receives the output of the decoders 204, 206, 208. In some implementations, the Softmax layer is separate from the decoders 204, 206, 208 and processes the outputs, y_(r), from the decoders 204, 206, 208. The output of the Softmax layer is then used in a beam search process to select orthographic elements. In some implementations, a Softmax layer is integrated within each of the decoders 204, 206, 208, such that the output y_(r) of the respective decoders 204, 206, 208 represents the output of the Softmax layer.

The decoders 204, 206, 208 are configured to generate, at each output step, a probability distribution over possible speech recognition hypotheses. Stated differently, for each of the decoders 204, 206, 208, the respective joint layer 240 generates, at each output step (e.g., time step), a probability distribution over possible speech recognition hypotheses. Here, the “possible speech recognition hypotheses” correspond to a set of output labels/symbols (also referred to as “speech units”) each representing a grapheme (e.g., symbol/character) or a word piece in a specified natural language. For example, when the natural language is English, the set of output labels may include twenty-seven (27) symbols, e.g., one label for each of the 26-letters in the English alphabet and one label designating a space. Accordingly, each respective joint layer 240 may output a set of values indicative of the likelihood of occurrence of each of a predetermined set of output labels. This set of values can be a vector (e.g., a one-hot vector) and can indicate a probability distribution over the set of output labels. In some cases, the output labels are graphemes (e.g., individual characters, and potentially punctuation and other symbols), but the set of output labels is not so limited. For example, the set of output labels can include wordpieces and/or entire words, in addition to or instead of graphemes. The output labels could also be other types of speech units, such as phonemes or sub-phonemes. The output distribution of the joint layer 240 can include a posterior probability value for each of the different output labels. Thus, if there are 100 different output labels representing different graphemes or other symbols, the output of the joint layer 240 can include 100 different probability values, one for each output label. The probability distribution can then be used to select and assign scores to candidate orthographic elements (e.g., graphemes, wordpieces, and/or words) in a beam search process (e.g., by the Softmax layer) for determining the transcription 120.

Within each of the decoders 204, 206, 208, the respective prediction network 250 may have two 2,048-dimensional LSTM layers, each of which is also followed by 640-dimensional projection layer, such that the LSTM-based prediction network may have about 23.4 million parameters. In other words, each prediction network 250 may include LSTM layers. In other configurations, the prediction network 250 may instead include conformer or transformer layers in lieu of LSTM layers. In yet other configurations, the prediction network 250 includes a V2 embedding look up table that includes an embedding prediction network. At each time step, the V2 embedding lookup table may receive, as input, the previous two predictions (e.g., 1-hot vectors) output by the prediction network 250, compute a respective embedding d₁, d₂ for each of the previous two predictions, and provide a concatenated output [d₁, d₂] to the joint layer 240. Comparatively, the V2 embedding lookup table may have only about two (2) million parameters, whereas an LSTM-based prediction network may include about 23.4 million parameters. Finally, the prediction network 250 may also be a one-layer neural network with 640 hidden units. The Softmax layer may be composed of a unified word piece or grapheme set that is generated using all unique word pieces or graphemes in a plurality of training utterances 132, 132 a-n (FIGS. 3A-3B). In some implementations, in order to limit the amount of future context that the ASR model 200 sees, the second encoder 220 uses some number of conformer layers (e.g., two layers) with a particular amount of right context (e.g., five seconds of right context), while the first encoder 210 continues to use LSTM layers. For these implementations, each conformer layer in the second encoder 220 may have 640 units to match the LSTM layers and adds around 10 million additional parameters.

In some implementations, where the model 200 operates in the streaming mode (i.e., the one-pass model), funnel pooling is used to down-sample the input acoustic frames 110, thereby reducing the number of parameters in the model 200 while preserving performance of the model 200. For example, a self-attention layer of the first encoder 210 receives, as input, a feature map x ∈ R^(T×D), where T is the original sequence length and D is the feature dimension. Thereafter, a down-sampled sequence, x′ ∈ R^(T′×D) is generated by average pooling, which is expressed as follows:

$\begin{matrix} {\text{x}^{\prime} = \text{AvgPool}\left( \text{x} \right)} & \text{­­­(1)} \end{matrix}$

Where T′ = T/2. Rather than providing x′ as input to the self-attention layer, it is used as query vector q, where the key vector k and the value vector ν are based on the input feature map x which maps to y ∈ R^(T′×D) as the output features of the self-attention layer as follows:

$\begin{matrix} {y = SelfAttention\left( {q = x^{\prime},kv = x} \right)} & \text{­­­(2)} \end{matrix}$

Continuing with the example in FIG. 2A, in some implementations, the model 200 a operates in both the streaming and non-streaming modes in parallel. When operating in both streaming and non-streaming mode at the same time, the model 200 a first performs streaming speech recognition on the audio data 110 using the first encoder 210 to generate the first higher order representation e^(S) for both the second encoder 220 and the first decoder 204. The first decoder 204 then produces the partial speech recognition results 120, 120 a based on the first higher order representation e^(S). The model 200 b also performs non-streaming speech recognition on the encoded audio data 110 where the second encoder 220 uses the first higher order representation e^(S) received from the first encoder 210 to generate the second higher order representation e^(a). The second decoder 206 then produces the final speech recognition result 120, 120 b based on the second higher order representation e^(a). As noted by the time, the first decoder 204 outputs the partial speech recognition results 120 a in a streaming fashion using the output from the first encoder 210, and then the model 200 a waits until the utterance is complete before operating in the non-streaming mode where the second decoder 206 uses the output from the second encoder 220 to produce the final speech recognition result 120 b. Thus, the final speech recognition result 120 b for the input utterance 106 may be delayed from the partial speech recognition results 120 a.

Referring to FIG. 2B, in some implementations, the model 200 b operates only in the streaming mode (i.e., a single-pass mode). This may occur, for instance, when the user 104 is using applications such as voice-search or on-device dictation, which require as little latency as possible. Here, the model 200 b performs streaming speech recognition on the audio data 110 using only the first encoder 210 to generate the first higher order feature representation e^(S) for the first decoder 204. The first decoder 204 then produces the partial speech recognition results 120, 120 a. Because the streaming mode of the model 200 b produces the partial speech recognition results 120, 120 a quickly, the inaccuracy of the term “play” is generally acceptable to users 104.

Referring to FIG. 2C, in some implementations the model 200 c operates only in the non-streaming mode (i.e., a two-pass model). The non-streaming mode may occur, for instance, in non-latency intensive applications such as when the user 104 is viewing a transcription of a voicemail left on his/her phone (e.g., FIG. 1B). As discussed above, this type of application benefits from using future context to improve performance in exchange for increased processing times. Here, the model 200 c uses the first encoder 210 to generate the first higher order feature representation e^(S) at each time step for input to the second encoder 220, but the first decoder 204 does not decode any of the first higher order feature representations e^(S). The model 200 c then performs non-streaming speech recognition on all of the audio data 110 where the second encoder 220 uses the first higher order feature representation e^(S) received from the first encoder 210 to generate the second higher order feature representation e^(a). The second decoder 206 then produces the final speech recognition result 120, 120 b. Since producing streaming speech recognition in real-time has little value to the user and latency is not a factor, the model 200 c may simply operate in only the non-streaming mode to produce the final speech recognition result 120, 120 b.

Referring to FIG. 2D, the model 200 can be generalized to a three-pass model 200 d (i.e., a large model) that operates only in the non-streaming mode. This non-streaming mode may occur, for instance, in non-latency intensive applications where a user 104 expects greater accuracy in speech recognition from the model 200. As discussed above, this type of application benefits from using an increased amount of future context at the expense of increased latency for the final speech recognition result 120, 120 b. Here, the model 200 d uses the first encoder 210 to generate the first higher order feature representation e^(S) at each time step for input to the second encoder 220, however the first decoder 204 does not decode any of the first higher order feature representations e^(S). The model 200 d next performs non-streaming speech recognition on all of the audio data 110 where the second encoder 220 uses the first higher order feature representations e^(S) received from the first encoder 210 to generate the second higher order feature representations e^(a). Unlike the model 200 c, the second decoder 206 does not decode any of the second higher order feature representations e^(a). Rather, the model 200 d again performs non-streaming speech recognition on all of the audio data 110 where the third encoder 230 uses the second higher order feature representation e^(a) received from the second encoder 220 to generate the third higher order feature representation e^(r). The third decoder 208 then produces the final speech recognition result 120, 120 b.

FIG. 3A and3B show an example of a two-stage training process 300, 300 a-b for training the model 200 to dynamically operate between streaming and/or non-streaming modes of varying model sizes. In some configurations, the training process 300 executes on the remote computing device 60 of FIGS. 1A and 1B. The training process 300 obtains a plurality of training utterances 132, 132 a-n stored in a sample database 130 and trains the model 200 on the training utterances 132 using a two-stage process. The sample database 130 may reside on the memory hardware of the remote computing device 60. As discussed above with respect to FIG. 2A, the first encoder 210, the second encoder 220, and the third encoder 230 can be trained jointly or together, simplifying the training process 300. Training the encoders 210, 220, 230 jointly means that the non-streaming encoder 220 may be trained directly on the output of the streaming encoder 210 (e.g., the first higher order representation e^(S)) instead of on input acoustic features (e.g., input acoustic frames 110), while the non-streaming encoder 230 may be trained directly on the output of the non-streaming encoder 220 (e.g., the second higher order feature representation e^(a)) instead of on input acoustic features (e.g., input acoustic frames 110). Advantageously, this ensures that that the model 200 learns to operate in either streaming or non-streaming modes during inference.

As shown in FIGS. 3A and 3B, there are three processing paths for the model 200, one for the streaming mode of the model 200 b (shown in FIG. 2B), one for the non-streaming mode of the model 200 c (shown in FIG. 2C), and one for the non-streaming mode of the model 200 d (shown in FIG. 2D). Referring to FIG. 3A, the training process 300 employs a first-stage training process 300 a. In the first stage, the training process 300 a uses cross-entropy training by forwarding the same mini-batch of training utterances 132 through each of the cascaded encoder models 200 b, 200 c, 200 d and linearly combines the losses for each cascaded encoder model 200 b, 200 c, 200 d with weights that sum to 1. In the first stage of the training process 300 a, jointly training the cascaded encoders 210, 220, 230 includes minimizing the weighted sum of the loss between the input processing paths.

Because there are three input processing paths within training process 300, the model’s loss includes three loss functions. Specifically, the loss for the streaming mode of the model 200 b is generally defined as a summation of the negative log probabilities corresponding to the probability distribution over possible speech recognition hypotheses given the input training utterances 132. That is, the model loss from the first encoder 210 connection to the decoder 204 is defined as, _(s) = - Σ_({(x→es) _(,y)})logP(y|e^(s)). The model loss for the non-streaming mode (e.g., model 200 c) is also generally defined as a summation of the negative log probabilities corresponding to the probability distribution over possible speech recognition hypotheses given the input training utterances 132. Therefore, the model loss from the second encoder 220 connection to the decoder 204 is defined as _(a) = - Σ_({(x→ea) _(,y)})logP(y|e^(a)). The model loss for the non-streaming mode (e.g., model 200 d) is also generally defined as a summation of the negative log probabilities corresponding to the probability distribution over possible speech recognition hypotheses given the input training utterances 132. Therefore, the model loss from the third encoder 230 connection to the third decoder 206 is defined as _(r) = - Σ_({(x→er) _(,y)})logP(y|e^(r)). In some implementations, where the model 200 only includes the encoders 210, 220 and the decoders 204, 206, the total loss between the two input paths for the first decoder 204 and the second decoder 206 is computed as a weighted sum of each input path loss = λ_(s) + (1 - λ)_(a) where λ is the weighting term.

In the second stage, to fine-tune the model 200 initialized during the first training stage 300 a using cross-entropy training, the training process 300 b performs discriminative training. Here, the discriminative training includes fine-tuning the model 200 using minimum word error rate (MWER) criteria. During the second stage of the training process 300 b, for each training utterance 132, training can occur in any of the streaming or non-streaming paths/passes. In other words, the input processing path is stochastically chosen as either training the model 200 b, or the model 200 c, or the model 200 d where the training process 300 b randomly samples each model 200 b, 200 c, 200 d with a probability equal to its loss weight. Here, the loss weight for the model 200 b may be 0.8, the loss weight of the model 200 c may be 0.15, and the loss weight of the model 200 d may be 0.05. By spending the most time training the path of the model 200 b, the accuracy of the model is increased without sacrificing performance in the later paths 200 c, 200 d. Thereafter, the respective decoder 204, 206, 208 of the sampled path performs beam searching on the training utterances 132 to generate an n-best list of hypotheses, and the negative log-likelihood is computed for all of the hypotheses and re-normalized in the n-best space to approximate the word error loss for minimization of loss in the model 200. Thus, for each hypothesis in the n-best list generated by the respective decoder for a corresponding training utterance, a respective number of word errors relative to a ground-truth transcription of the corresponding training utterance is identified and the training process uses the MWER criteria to minimize word error rate based on the respective number of word errors identified for each hypothesis in the n-best list for the corresponding training utterance 132. As used herein, the n-best list includes the n highest ranked hypotheses wherein each hypotheses in the n-best list includes a candidate transcription for the same corresponding training utterance 132. In some implementations, n is equal to four (4) such that the respective decoder 204, 206, 208 of the sampled path performs beam searching on the training utterances 132 to generate the top-4 hypotheses and the negative log-likelihood is computed for all the hypotheses (greater than 4) of the hypotheses and re-normalized in the top-4 space. Moreover, by sampling the training utterances 132, the training process only needs to calculate the loss once for each training utterance 132 at each pass/path, which greatly speeds up the second stage of the training process 300 b. In some implementations, where a longer training time is tolerated, an alternative training process is employed to train each input processing path/pass with each training utterance and compute both the loss of the model 200 b and the model 200 c for each training utterance 132.

In the example shown in FIG. 3A, the training utterances 132 b, 132 c are chosen to train each of the processing paths represented by the cascaded encoders models 200 b, 200 c, 200 d in the first stage of the training process 300 a.The cascaded encoders model 200 b receives the training utterances 132 b, 132 c, and the first encoder 210 converts the training utterances 132 b, 132 c into the first higher order feature representations (e.g., audio embeddings) as output. The first decoder 204 then receives the first higher order feature representations of training utterances 132 b, 132 c as input and generates an output which is tested for its accuracy. Similarly, training utterances 132 b, 132 c are chosen to train the second processing path represented by the cascaded encoders model 200 c. The cascaded encoders model 200 c receives the training utterances 132 b, 132 c, and the first encoder 210 converts the training utterances 132 b, 132 c into the first higher order feature representations (e.g., audio embeddings) as output. The second encoder 220 receives the first higher order feature representations of training utterances 132 b, 132 c as input and generates second higher order feature representations of the training utterances 132 b, 132 c as output. The second decoder 206 then receives the second higher order feature representations of training utterances 132 b, 132 c as input and generates an output which is tested for its accuracy. Likewise, the cascaded encoders model 200 d receives the training utterances 132 b, 132 c, and the first encoder 210 converts the training utterances 132 b, 132 c into the first higher order feature representations (e.g., audio embeddings) as output. The second encoder 220 receives the first higher order feature representations of training utterances 132 b, 132 c as input and generates second higher order feature representations of the training utterances 132 b, 132 c as output. The third encoder 230 receives the second higher order feature representations of training utterances 132 b, 132 c as input and generates third higher order feature representations of training utterances 132 b, 132 c as output. The third decoder 208 then receives the third higher order feature representations of training utterances 132 b, 132 c as input and generates an output which is tested for its accuracy.

As shown in FIG. 3B, in the second stage of the training process 300 b, the training utterances 132 used by the training process 300 b are randomly selected, and each pass of the model 200 is randomly selected at a probability equal to the loss weight assigned to each path of the model 200. Based on its loss weight, the cascaded encoders model 200 b receives the training utterances 132 a, 132 b, 132 c, and the first encoder 210 converts the training utterances 132 a, 132 b, 132 c into the first higher order feature representations (e.g., audio embeddings) as output. The first decoder 204 then receives the first higher order feature representations of training utterances 132 a, 132 b, 132 c as input and generates an output which is tested for its accuracy. Based on its loss weight, the cascaded encoders model 200 c receives the training utterances 132 b, 132 d, and the first encoder 210 converts the training utterances 132 b, 132 d into the first higher order feature representations (e.g., audio embeddings) as output. The second encoder 220 receives the first higher order feature representations of training utterances 132 b, 132 d as input and generates second higher order feature representations of the training utterances 132 b, 132 d as output. The second decoder 206 then receives the second higher order feature representations of training utterances 132 b, 132 d as input and generates an output which is tested for its accuracy. Based on its loss weight, the cascaded encoders model 200 d receives the training utterance 132 c, and the first encoder 210 converts the training utterance 132 c into the first higher order feature representation (e.g., audio embeddings) as output. The second encoder 220 receives the first higher order feature representations of training utterance 132 c as input and generates second higher order feature representations of the training utterance 132 c as output. The third encoder 230 receives the second higher order feature representations of training utterance 132 c as input and generates third higher order feature representations of training utterance 132 c as output. The third decoder 208 then receives the third higher order feature representations of training utterance 132 c as input and generates an output which is tested for its accuracy.

FIG. 4 includes a flowchart of an example arrangement of operations for a method 400 of performing streaming and non-streaming speech recognition using an ASR model 200. At operation 402, the method 400 includes receiving, as input to the ASR model 200, a sequence of acoustic frames 110. At operation 404, the method 400 also includes generating, by a first encoder 210, at each of a plurality of output steps, a first higher order feature representation for a corresponding acoustic frame 110 in the sequence of acoustic frames 110. At operation 406, the method 400 includes receiving, as input to both a first decoder 204 and a second encoder 220, the first higher order feature representation generated by the first encoder 210 at each of the plurality of output steps.

The method 400 also includes, at operation 408, generating, by the second encoder 220, at each of the plurality of output steps, a second higher order feature representation for a corresponding first higher order feature representation. At operation 410, the method 400 includes generating, by the first decoder 204, at each of the plurality of output steps, a first probability distribution over possible speech recognition hypotheses. The method 400 further includes, at operation 412, receiving, as input to a second decoder 206, the second higher order feature representation generated by the second encoder 220 at each of the plurality of output steps. At operation 414, the method 400 also includes generating, by the second decoder 206, at each of the plurality of output steps, a second probability distribution over possible speech hypotheses.

FIG. 5 is schematic view of an example computing device 500 that may be used to implement the systems (e.g., the audio subsystem 108, the ASR system 109, the user interface generator 107, and/or the model 200) and methods (e.g., the method 400) described in this document. The computing device 500 is intended to represent various forms of digital computers, such as laptops, desktops, workstations, personal digital assistants, servers, blade servers, mainframes, and other appropriate computers. The components shown here, their connections and relationships, and their functions, are meant to be exemplary only, and are not meant to limit implementations of the inventions described and/or claimed in this document.

The computing device 500 includes a processor 510 (e.g., data processing hardware), memory 520 (e.g., memory hardware), a storage device 530, a high-speed interface/controller 540 connecting to the memory 520 and high-speed expansion ports 550, and a low speed interface/controller 560 connecting to a low speed bus 570 and a storage device 530. Each of the components 510, 520, 530, 540, 550, and 560, are interconnected using various busses, and may be mounted on a common motherboard or in other manners as appropriate. The processor 510 (e.g., data processing hardware 12, 62 of FIGS. 1A-1B) can process instructions for execution within the computing device 500, including instructions stored in the memory 520 or on the storage device 530 to display graphical information for a graphical user interface (GUI) on an external input/output device, such as display 580 coupled to high speed interface 540. In other implementations, multiple processors and/or multiple buses may be used, as appropriate, along with multiple memories and types of memory. Also, multiple computing devices 500 may be connected, with each device providing portions of the necessary operations (e.g., as a server bank, a group of blade servers, or a multi-processor system).

The memory 520 (e.g., memory hardware 14, 64 of FIGS. 1A-1B) stores information non-transitorily within the computing device 500. The memory 520 may be a computer-readable medium, a volatile memory unit(s), or non-volatile memory unit(s). The non-transitory memory 520 may be physical devices used to store programs (e.g., sequences of instructions) or data (e.g., program state information) on a temporary or permanent basis for use by the computing device 500. Examples of non-volatile memory include, but are not limited to, flash memory and read-only memory (ROM) /programmable read-only memory (PROM) / erasable programmable read-only memory (EPROM) / electronically erasable programmable read-only memory (EEPROM) (e.g., typically used for firmware, such as boot programs). Examples of volatile memory include, but are not limited to, random access memory (RAM), dynamic random access memory (DRAM), static random access memory (SRAM), phase change memory (PCM) as well as disks or tapes.

The storage device 530 is capable of providing mass storage for the computing device 500. In some implementations, the storage device 530 is a computer-readable medium. In various different implementations, the storage device 530 may be a floppy disk device, a hard disk device, an optical disk device, or a tape device, a flash memory or other similar solid state memory device, or an array of devices, including devices in a storage area network or other configurations. In additional implementations, a computer program product is tangibly embodied in an information carrier. The computer program product contains instructions that, when executed, perform one or more methods, such as those described above. The information carrier is a computer- or machine-readable medium, such as the memory 520, the storage device 530, or memory on processor 510.

The high speed controller 540 manages bandwidth-intensive operations for the computing device 500, while the low speed controller 560 manages lower bandwidth-intensive operations. Such allocation of duties is exemplary only. In some implementations, the high-speed controller 540 is coupled to the memory 520, the display 580 (e.g., through a graphics processor or accelerator), and to the high-speed expansion ports 550, which may accept various expansion cards (not shown). In some implementations, the low-speed controller 560 is coupled to the storage device 530 and a low-speed expansion port 590. The low-speed expansion port 590, which may include various communication ports (e.g., USB, Bluetooth, Ethernet, wireless Ethernet), may be coupled to one or more input/output devices, such as a keyboard, a pointing device, a scanner, or a networking device such as a switch or router, e.g., through a network adapter.

The computing device 500 may be implemented in a number of different forms, as shown in the figure. For example, it may be implemented as a standard server 500a or multiple times in a group of such servers 500 a, as a laptop computer 500 b, or as part of a rack server system 500 c.

Various implementations of the systems and techniques described herein can be realized in digital electronic and/or optical circuitry, integrated circuitry, specially designed ASICs (application specific integrated circuits), computer hardware, firmware, software, and/or combinations thereof. These various implementations can include implementation in one or more computer programs that are executable and/or interpretable on a programmable system including at least one programmable processor, which may be special or general purpose, coupled to receive data and instructions from, and to transmit data and instructions to, a storage system, at least one input device, and at least one output device.

A software application (i.e., a software resource) may refer to computer software that causes a computing device to perform a task. In some examples, a software application may be referred to as an “application,” an “app,” or a “program.” Example applications include, but are not limited to, system diagnostic applications, system management applications, system maintenance applications, word processing applications, spreadsheet applications, messaging applications, media streaming applications, social networking applications, and gaming applications.

These computer programs (also known as programs, software, software applications or code) include machine instructions for a programmable processor, and can be implemented in a high-level procedural and/or object-oriented programming language, and/or in assembly/machine language. As used herein, the terms “machine-readable medium” and “computer-readable medium” refer to any computer program product, non-transitory computer readable medium, apparatus and/or device (e.g., magnetic discs, optical disks, memory, Programmable Logic Devices (PLDs)) used to provide machine instructions and/or data to a programmable processor, including a machine-readable medium that receives machine instructions as a machine-readable signal. The term “machine-readable signal” refers to any signal used to provide machine instructions and/or data to a programmable processor.

The processes and logic flows described in this specification can be performed by one or more programmable processors, also referred to as data processing hardware, executing one or more computer programs to perform functions by operating on input data and generating output. The processes and logic flows can also be performed by special purpose logic circuitry, e.g., an FPGA (field programmable gate array) or an ASIC (application specific integrated circuit). Processors suitable for the execution of a computer program include, by way of example, both general and special purpose microprocessors, and any one or more processors of any kind of digital computer. Generally, a processor will receive instructions and data from a read only memory or a random access memory or both. The essential elements of a computer are a processor for performing instructions and one or more memory devices for storing instructions and data. Generally, a computer will also include, or be operatively coupled to receive data from or transfer data to, or both, one or more mass storage devices for storing data, e.g., magnetic, magneto optical disks, or optical disks. However, a computer need not have such devices. Computer readable media suitable for storing computer program instructions and data include all forms of non-volatile memory, media and memory devices, including by way of example semiconductor memory devices, e.g., EPROM, EEPROM, and flash memory devices; magnetic disks, e.g., internal hard disks or removable disks; magneto optical disks; and CD ROM and DVD-ROM disks. The processor and the memory can be supplemented by, or incorporated in, special purpose logic circuitry.

To provide for interaction with a user, one or more aspects of the disclosure can be implemented on a computer having a display device, e.g., a CRT (cathode ray tube), LCD (liquid crystal display) monitor, or touch screen for displaying information to the user and optionally a keyboard and a pointing device, e.g., a mouse or a trackball, by which the user can provide input to the computer. Other kinds of devices can be used to provide interaction with a user as well; for example, feedback provided to the user can be any form of sensory feedback, e.g., visual feedback, auditory feedback, or tactile feedback; and input from the user can be received in any form, including acoustic, speech, or tactile input. In addition, a computer can interact with a user by sending documents to and receiving documents from a device that is used by the user; for example, by sending web pages to a web browser on a user’s client device in response to requests received from the web browser.

A number of implementations have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the disclosure. Accordingly, other implementations are within the scope of the following claims. 

What is claimed is:
 1. An automated speech recognition (ASR) model comprising: a first encoder configured to: receive, as input, a sequence of acoustic frames; and generate, at each of a plurality of output steps, a first higher order feature representation for a corresponding acoustic frame in the sequence of acoustic frames; a first decoder configured to: receive, as input, the first higher order feature representation generated by the first encoder at each of the plurality of output steps; and generate, at each of the plurality of output steps, a first probability distribution over possible speech recognition hypotheses; a second encoder configured to: receive, as input, the first higher order feature representation generated by the first encoder at each of the plurality of output steps; and generate, at each of the plurality of output steps, a second higher order feature representation for a corresponding first higher order feature frame; and a second decoder configured to: receive, as input, the second higher order feature representation generated by the second encoder at each of the plurality of output steps; and generate, at each of the plurality of output steps, a second probability distribution over possible speech recognition hypotheses.
 2. The ASR model of claim 1, wherein the first decoder is further configured to generate partial speech recognition results based on the first probability distribution over possible speech recognition hypotheses.
 3. The ASR model of claim 1, wherein the first encoder comprises a causal encoder comprising one of: a plurality of unidirectional long short-term memory (LSTM) layers; a plurality of conformer layers; or a plurality of transformer layers.
 4. The ASR model of claim 1, wherein the second encoder comprises a non-causal encoder comprising one of: a plurality of unidirectional long short-term memory (LSTM) layers; a plurality of conformer layers; or a plurality of transformer layers.
 5. The ASR model of claim 1, wherein the first decoder comprises: a prediction network configured to: receive, as input, a sequence of non-blank symbols output by a final softmax layer; and generate, at each of the plurality of output steps, a dense representation; and a joint network configured to: receive, as input, the dense representation generated by the prediction network at each of the plurality of output steps and the first higher order feature representation generated by the first encoder at each of the plurality of output steps; and generate, at each of the plurality of output steps, the first probability distribution over possible speech recognition hypotheses.
 6. The ASR model of claim 5, wherein the prediction network comprises: a long short-term memory (LSTM)-based prediction network; or a V2 embedding look-up table.
 7. The ASR model of claim 1, wherein the second decoder comprises: a prediction network configured to: receive, as input, a sequence of non-blank symbols output by a final softmax layer; and generate, at each of the plurality of output steps, a dense representation; and a joint network configured to: receive, as input, the dense representation generated by the prediction network at each of the plurality of output steps and the second higher order feature representation generated by the second encoder at each of the plurality of output steps; and generate, at each of the plurality of output steps, the second probability distribution over possible speech recognition hypotheses.
 8. The ASR model of claim 7, wherein the prediction network comprises: a long short-term memory (LSTM)-based prediction network; or a V2 embedding look-up table.
 9. The ASR model of claim 1, wherein the first encoder comprises a greater number of parameters than the second encoder.
 10. The ASR model of claim 1, further comprising: a third encoder configured to: receive, as input, the second higher order feature representation generated by the second encoder at each of the plurality of output steps; and generate, at each of the plurality of output steps, a third higher order feature representation for a corresponding second higher order feature representation; and a third decoder configured to: receive, as input, the third higher order feature representation generated by the third encoder at each of the plurality of output steps; and generate, at each of the plurality of output steps, a third probability distribution over possible speech recognition hypotheses.
 11. A computer-implemented method that when executed on data processing hardware causes the data processing hardware to perform operations comprising: receiving a sequence of acoustic frames; and generating, by a first encoder, at each of a plurality of output steps, a first higher order feature representation for a corresponding acoustic frame in the sequence of acoustic frames; generating, by a second encoder, at each of the plurality of output steps, a second higher order feature representation for a corresponding first higher order feature representation; generating, by a first decoder, at each of the plurality of output steps, a first probability distribution over possible speech recognition hypotheses; and generating, by a second decoder, at each of the plurality of output steps, a second probability distribution over possible speech recognition hypotheses.
 12. The computer-implemented method of claim 11, wherein the operations further comprise generating partial speech recognition results based on the first probability distribution over possible speech recognition hypotheses.
 13. The computer-implemented method of claim 11, wherein the first encoder comprises a causal encoder comprising one of: a plurality of unidirectional long short-term memory (LSTM) layers; a plurality of conformer layers; or a plurality of transformer layers.
 14. The computer-implemented method of claim 11, wherein the second encoder comprises a non-causal encoder comprising one of: a plurality of unidirectional long short-term memory (LSTM) layers; a plurality of conformer layers; or a plurality of transformer layers.
 15. The computer-implemented method of claim 11, wherein the operations further comprise, at each of the plurality of output steps: generating, by a prediction network of the first decoder based on a sequence of non-blank symbols output by a final softmax layer, a dense representation; and generating, by a joint network of the first decoder, the first probability distribution over possible speech recognition hypotheses based on the dense representation generated by the prediction network.
 16. The computer-implemented method of claim 15, wherein the prediction network of the first decoder comprises: a long short-term memory (LSTM)-based prediction network; or a V2 embedding look-up table.
 17. The computer-implemented method of claim 11, wherein the operations further comprise, at each of the plurality of output steps: generating, by a prediction network of the second decoder, at each of the plurality of output steps, a dense representation; receiving, as input to a joint network of the second decoder, the dense representation generated by the prediction network at each of the plurality of output steps and the second higher order feature representation generated by the second encoder at each of the plurality of output steps; and generating, by the joint network of the second decoder, at each of the plurality of output steps, the second probability distribution over possible speech recognition hypotheses.
 18. The computer-implemented method of claim 17, wherein the prediction network of the second decoder comprises: a long short-term memory (LSTM)-based prediction network; or a V2 embedding look-up table.
 19. The computer-implemented method of claim 11, wherein the first encoder comprises a greater number of parameters than the second encoder.
 20. The computer-implemented method of claim 11, wherein the operations further comprise, while performing the streaming speech recognition and the non-streaming speech recognition on the sequence of acoustic frames: receiving, as input to a third encoder, the second higher order feature representation generated by the second encoder at each of the plurality of output steps; generating, by the third encoder, at each of the plurality of output steps, a third higher order feature representation for a corresponding second higher order feature representation; receiving, as input to a third decoder, the third higher order feature representation generated by the third encoder at each of the plurality of output steps; and generating, by the third decoder, at each of the plurality of output steps, a third probability distribution over possible speech recognition hypotheses. 